It is known in the art of tool mounting to dispose a changeable cutting tool in a drivable tool holder provided as part of a cutting machine, for example, a drill press, milling machine, hollow milling machine, lathe, punch press, or the like. In rotating applications, ideally the tool is a) perfectly centered on the axis of rotation of the holder, which is the machine spindle axis, and b) restrained by positive restraint means to prevent the tool from being turned within the holder as a result of torque loads imposed during cutting operations. Unfortunately, in the prior art either of these criteria may be met, to a high degree of success, but typically not both.
In the known art, a tool having a cylindrical shank may be restrained from turning by a tool holder or chuck having three radially-retractable equilaterally-arranged jaws that grip the tool by exerting radial force. For even more positive anti-rotation means, the tool shank may be formed hexagonally rather than cylindrically, or may be provided with one or more flats for receiving one or more set screws in the holder. Such mounting means can provide a high level of rotational restraint but inadequate centering capability for high-precision machining.
For high-precision centering in the known art, such that the tool axis is concentric with the machine spindle, one approach is the use of a shrink fit tool holder that provides an interference fit between the shank of the cutting tool and internal diameter of a cylindrical aperture in the tool holder. The fit may be enhanced by forming the aperture at an inner diameter slightly less than the outer diameter of the tool shank, and then heating the holder to increase the inner diameter such that it will accept the tool. When the holder is again cool, the tool is firmly gripped within the aperture. See U.S. Pat. No. 5,582,494 ('494) issued Dec. 10, 1996 to Cook, which is incorporated herein by reference. When the tool is assembled into the holder in this fashion, recited in the '494 patent as a second embodiment, the tool will run true to the internal diameter of the holder provided that the holder is accurately manufactured to a high level of concentricity between the tool aperture and a shank of the tool holder for mounting onto the machine itself.
The '494 patent also discloses as a first embodiment the use of an axial backing screw arrangement within the tool holder for setting the milling height of the tool, and also an angularly inclined flat on the tool, engageable by an angled set screw in the tool holder, for securing the tool against the backing screw, ensuring the set height, and preventing the tool from backing out of the holder aperture.
The '494 patent teaches and claims in claim 1 that the tool may be rotationally restrained within the aperture solely by friction between the tool shank and aperture wall. Thus, the '494 patent specifically teaches away from using additional means for positively restraining the tool against rotation in the holder. However, experience in using tools and tool holders in accordance with this invention has shown that shrink fitting alone can be inadequate to restrain the tool from rotation within the tool holder under some high torque loads. We have found that a positive means for restraining rotation can be useful, and in fact necessary, in combination with shrink-fit centering of the tool in sustaining high torque loads on the tool.
What is needed in the art is a means for mounting a tool in a tool holder wherein the tool axis has a high degree of concentricity with the tool holder axis and wherein the tool is simultaneously positively restrained from rotation within the tool holder.